The present invention relates to additives to foundry sand molding aggregates used in the manufacture of molds and cores, and more particularly an additive containing a high percentage of titanium dioxide (TiO.sub.2) which improves the quality of castings by reducing veining and gas defects.
Iron oxides have been used for years in foundry applications to improve core properties and the quality of castings. Iron oxides have proven to be advantageous as an additive to foundry sand molding aggregates, which in turn improves the quality of castings, by reducing the formation of thermal expansion defects, such as veining, scabs, buckles, and rat tails as well as gas defects, such as pinholes and metal penetration.
There are several iron oxides which are currently used in foundries today. These include red iron oxide, also known as hematite (Fe.sub.2 O.sub.3), black iron oxide, also known as magnetite (Fe.sub.3 O.sub.4) and yellow ochre. Another iron oxide which is presently being used is Sierra Leone concentrate which is a hematite ore black in color. Red iron oxide and black iron oxide are the most popular iron oxides in use. Red iron oxide typically includes 60-87% Fe.sub.2 O.sub.3, 7.5-8.5% silica dioxide (SiO.sub.2), 2-9.5% alumina dioxide (AlO.sub.2), 0.1-11% calcium oxide (CaO), 0.2-2.6% magnesium oxide (MgO) and 0.2-0.4% manganese oxide (MnO). Black iron oxide includes 60-64% Fe.sub.3 O.sub.4, 1-2% SiO.sub.2, 3-4% AlO.sub.2, 0.1-0.2% CaO.sub.2, 0.8-1% MgO, 0.05-0.1% MnO. Also black iron oxide may contain about 4-6% titanium dioxide. Yellow ochre includes 50-60% Fe.sub.2 O.sub.3, 19-21% SiO.sub.2, 5-7% AlO.sub.2 and 0.5-0.7% CaO. Sierra Leone concentrate ore typically includes 92.5% Fe.sub.2 O.sub.3, 4.75% SiO.sub.2, 1% Al.sub.2 O.sub.3, 0.16% CaO, and 0.73% MnO.
The currently accepted method of employing the above iron oxides is to add approximately 1-3% by weight to the sand mold aggregates during mixing. The exact mechanism by which iron oxides improve surface finish in not totally understood. However, it is generally believed that the iron oxides increase the hot plasticity of the mixture by the formation of a glassy layer between the grains which deforms and "gives", without fracturing at metallurgical temperatures, to prevent fissures from opening up in the sand, which in turn reduces veining.
It should be noted that additives containing high titanium dioxide compositions have not been intentionally added to foundry molding aggregates. Although black iron oxide may contain about 4-6% titanium dioxide, the amount of titanium dioxide in the sand molding aggregate is extremely small. As seen from above, the amount of titanium dioxide typically added to the sand molding aggregate is only about 0.0003% to about 0.0009% by weight of the entire aggregate mixture. Currently, titanium dioxide is used in the manufacture of pigment in the paint industry or as a source of titanium metal itself. The use of high titanium dioxide containing materials as an additive to foundry aggregates thus is a unique application of this mineral.
U.S. Patents to Aignesberger et al U.S. Pat. No. 3,661,829, Dittrich et al U.S. Pat. No. 4,211,567 and Brugger U.S. Pat. No. 4,343,345 are of limited interest in this regard since each refers to the use of an oxide of titanium in sand molding processing. Also, Klaudinyi et al U.S. Pat. No. 3,793,284 refers the use of fire clay or ball clays as an additive to foundry core pastes. However, none of the above references refer to or discuss the advantageous effects of an additive composed of a high percentage of titanium dioxide (TiO.sub.2).